Mean frequency control of frequency-modulated oscillators



Feb. 26, 1952 FERNSLER j 2,587,303

MEAN FREQUENCY CONTROL OF FREQUENCY-'MODULATED OSCILLATORS Filed-Sept. 6, I946 1/ 4 5 Pan ii JUPFLy INVENTOR fieorye LlZr/zsler BY WM,

ATTO R N EY Patented Feb. 26, 1952 UNI TE D' S TAT E S MEAN FREQUENCY CONTROL OFfFRE- QUENGY-MODULATED OSCILLATORS of Delaware Application September 6, 1946, Serial No. 695,307

(Cl.v 250-136) This invention relates tofrequency modulated oscillators and particularly those utilizing magnetrons'; klystrons; and similartubes embodying orfused with' resonant cavity-structures.

During the operation of oscillators'of the reso nant cavity type, the variations ofambient conditionssuch as atmospheric temperature or pressure; foFexample, directly or indirectly cause the physical dimensions'of the'cavity structure to change"withponsequentshift of the band of frequenciessweptby' the'osc'illator. Airborne equipmentis particularly subject to "this fault. It is theiprincipal object of this invention to maintain themid-band frequency-of a frequency-modulatedi'oscillator" substantially constant notwithstanding such variations;

In accordance" with the -invention, a temperature' responsive element" such" as a oi-metallic strip; 'Bourdon tube, an" expansiblebel-lows or equivalentis disposed ingood heat-transfer relation with the resonant cavity structure and the movementsof"the-'heat responsive device in response to" temperature changes'of the cavity'are utili'zedsmoothly 'to. vary loss of "heat generated Within the tube inproper sense to restore the mid-band frequency to the" desired magnitude. Specifically and in'on'e'embodiment ofthe inven-- tion'," the heat=responsive-device'may be used to regulate the current supplied to the magnetron or equivalent tube by adjustment of a voltage dividing resistance in the I power supplyiof the tube.

The invention. further resides in methods and systems "having the" features of noveltydisclosed and claimed.

For more detailed understanding of the invention' and for illustration of embodiments thereof, reference is made to' the-"accompanying drawings in .which:

Figurel schematically illustrates-a frequencymodu'lated magnetron-and expansible bellows arrangernents for regulating supply of current thereto;

Figures 2 and 3 schematically illustrate modifications of the system shown in Figure 1.

Referring to Figure l, the tube It is generically representative of an oscillator tube of type having-or used with resonant cavity structures for producing oscillations of ultra-high frequencies; Specifically, the tube It is a double-ended magnetron" comprising a heateror filament H disposed- -between the adjacent open ends of two resonant cavities l2 and 13' whose dimensions determine the frequencyof oscillations generated blF'the-tube: With; each of th'e caxriti'eshis associat'ed: a circular'cathode l 4 'andza collector ri-ng.

modulating frequency'gupon' voltage or currentsupplied to the tube :or' upon the magnetic field,

in the case of 1a magnetron;

In the particular arrangement shown in Figureil, frequency modulation'of'the oscillator I0 is effected by variation of the "impedance of the concentric'line lliwhich couples the oscillator to th'e :antenna' or otherul'oad circuit. At its input end, ithe'inner: conductor of line H is formed into a pick-up loop IB within the resonant cavity l2.

' Between thdinput and output ends of line H,

it is provided with a stub :line 19 Whose'refiected impedanceis varied in accordance with the modulating frequency. This may be effected meo'nam ically bya; device of the type shown mice-pending application'serial No. 601,798 J ohnson et al., now U.'- S. Patent No. 2,482,914, dated September 2'7, 1949; or by arectifierZil fedby the modulating voltag'e and serving as a variable termination of thestub line I9:

In any event, whether the frequency modulationis effected by thisarrangement or any other one, the range of frequencies swept by the oscillater is subject to changedependent upon conditions which directly or indirectly affect the di--- mensions of the cavity structures-of the oscillator. For example; a substantial change in atmospheric temperature modifies the rate at which heat is transferred from the cavities l2 and It with resultant change in their dimensionsand therefore their resonant frequency. Assuming the line or wave guide !'l' is air filled, a substantial change in atmospheric pressure will change the load im-' pedanc'e and therefore the heat 'dissipation'of the tube with' resultant changeof-the resonant irequency-of the cavity structure;

To' minimize the drift of the oscillator" fre= -quency -andconsequent impairment of--uti1 ity ofthe':systemiincludingrit, there is provided asuit-- able device. 2'I'- responsive to-changes in temperatun-e ofithelcavity structures Imthejor'm shownin Figure 1, the heat-responsive device 2| is an expansible bellows filled with suitable gas or vapor and whose base or reservoir is mounted upon the tube envelope [6 for rapid conduction of heat from the resonant cavity structure of tube ID to the gas or vapor contained in the bellows. The movable end of the bellows is suitably mechanically connected, generally as indicated by broken line 22 and member 23 of suitable electrical insulating material, to the movable voltage regulating contact 24 of a voltage-dividing re sistance 25 connected across the output terminals of a direct-current power supply 26 which furnishes the cross-beam current to the tube H3. Thus when the temperature of the oscillator tube It! changes the effective output voltage or current of the source 26 is changed in avoidance or reduction of departure from the desired midfrequency of the band swept by the oscillator.

In short, the effect of a rise in ambient temperature upon the cavity dimensions is compensated by decrease of the electrical input to the tube with consequent decrease of heat generated within the tube: conversely, any tendency for the cavity dimensions to change with fall of ambient temperature is offset by a compensating increase of the electrical input to the tube.

As the relation between the deviation of the mean frequency of the oscillator from its desired value and the change in cross-beam current necessary to restore the cavity dimensions to extent correcting for the frequency deviation is not ordinarily linear, it is usually necessary to taper the resistance 25 or to use a motion-transmitting arrangement which affords suitable unequal successive increments of adjustment of contact 24 for successive equal increments of movement of the free end of the bellows 2|. The required taper or mechanical linkage necessary to attain satisfactory compensation may be empirically determined for a particular oscillator by subjecting it to the anticipated range of temperatures and noting the required changes in cross-beam current necessary to restore the oscillator frequency to the proper mean frequency.

In the modification shown in Figure 2, the temperature-responsive device for producing mechanical movements in response to changes in temperature of the resonant cavity structure of the oscillator tube MA is a Bourdon tube 2IA whose base or reservoir is mounted for good transfer of heat by conduction from the resonant cavity structure of the oscillator to the expansible gas or vapor confined in the Bourdon tube system. The motion of the free end of the Bourdon tube is amplified and transmitted, through a suitable gear and shaft arrangement for example, to a disk 26 upon which is mounted a slidewire 25A supplied with current by a battery 21 or equivalent direct-current source. The stationary contact 25A of the slidewire is connected to the control grid, for example, of a regulator tube 28 included in the power supply 26A which may be of conventional type including for example stepup transformer 29, rectifier 3B, and a filter network including one or more inductances 3| and condensers 32. The slidewire 25A and battery 2? provide a bias for the tube 28 which varies as a function of the temperature of the resonant cathode structure of tube A. The tube 28 is used in the particular arrangement shown as a shunttype regulator: as its grid becomes less negative or more positive, it draws a greater current through the series resistor 33 and so lowers the supply voltage for the anode circuitoftube IDA.

Conversely, the eifective supply voltage is increased when the Bourdon tube adjusts slid wire 25A to make the grid of tube 28 more negative or less positive.

In short, as the dimensions of the cavity structure change, the grid-biasing voltage of the tube 28 is varied to change the effective output voltage for the source 26A in proper sense to avoid a continuing departure of the mid-band frequency of the oscillator from the desired frequency. As in the system of Fig. 1, the electrical input to the cavity oscillator, and therefore the heat generated therein, is varied inversely with ambient temperature to minimize any change in cavity dimensions.

This variation of supply voltage is slow compared with the variations superposed upon it by the sweep generator 34 or equivalent source of modulating frequency coupled to the anode circuit as by the condenser 35. For example, the frequency of the sweep generator may be of the order of cycles per second, whereas the change of the supply voltage due to the action of tube 28 is quite gradual because of the heat-capacity or thermal lag of the component structure of the oscillator tube.

In the modification shown in Figure 3, the device for translating any drift from mean frequency into a mechanical movement is a bi-metallic strip 2 l3 anchored at one end, for example, to the envelope of the oscillator tube IGB and having its free end suitably mechanically coupled by linlg age 2213 to the adjustable element of a variable resistance or speed-regulating rheostat 25B which controls the speed of the driving motor 36 of a blower 31 which directs a stream of air toward the oscillator tube. This arrangement differs from those of Figs. 1 and 2 in that the effect of changes in ambient temperature upon the cavity dimensions is compensated, not by varying the rate at which heat is generated in the tube, but by varying the rate at which heat is removed therefrom by forcible convection. In other respects, the system is similar to those previously discussed and requires no further explanation.

It shall be understood the invention is not limited to the particular arrangement shown but that changes and modifications may be made Within the scope of the appended claims.

I claim as my invention:

1. An arrangement for minimizing drift of the mean frequency of a frequency-modulated oscillator comprising a heat-responsive device in heat-transfer relation to the oscillator tube, and regulating means actuated by said device for varying loss of heat from said tube in sense to compensate for effect of change of an ambient condition upon the output frequency of the oscillator.

2. An arrangement for minimizing drift of the frequency of an oscillator utilizing a tube of the resonant cavity type, comprising a thermal-responsive device in heat-transfer relation to said tube cavity, and regulating means actuated by said device smoothly to vary the electrical input to the tube in sense to compensate for dimensional changes of its cavity structure.

3. An arrangement for minimizing drift of the frequency of an oscillator tube having resonant cavity structure whose dimensions are afiected by ambient conditions and by the electrical in put of the tube comprising a thermal-responsive device receiving heat by conduction from said tube cavity structure, and regulating means actuated by said device smoothly to vary the ,elec

trical input to said tube in sense to reduce desource of biasing voltage determining the crossbeam current between the cathode and collector of said magnetron, a thermal-responsive device in heat-transfer relation to cavity structure of said magnetron, a potentiometer having connections to said source and said collector, and means mechanically connecting said device to the adjustable element of said potentiometer to vary the effective magnitude of said biasing voltage in compensation of tendency of the frequency of the output of the magnetron to drift.

6. An arrangement for minimizing the drift with temperature of the frequency of an oscillator utilizing a tube of the resonant cavity type comprising a heat-responsive device in heattransfer relation to the cavity of the oscillator tube, means for varying the heat-loss from said oscillator cavity including a movable member, and means mechanically connecting said member to said device for adjustment thereby in compensation for tendency of the oscillator frequency to drift.

7. An arrangement for minimizing drift of the frequency generated by an oscillator tube of the resonant cavity type comprising a device responsive to the dimensional changes with operating temperature of the cavity structure, a source of supply voltage for said tube including control means operable to vary the effective output voltage of said source, and means operatively connecting said control means to said responsive device for adjustment of said supply voltage and therefore the heat-loss from the resonant cavity of said oscillator tube in compensation for said dimensional changes thereof.

8. An arrangement for minimizing drift of the frequency generated by an oscillator tube of the resonant cavity type comprising a device responsive to dimensional changes with operating temperature of the tube cavity structure, a source of operating voltage for said oscillator tubelcoinprising a regulator tube controlling the effective output voltage of said source, means adjustable to vary a biasing voltage for said regulator tube, and means operatively-connecting said adjustable means to said responsive device to effect variation of the effective regulated voltage of said source and therefore the heat-loss from said tube cavity structure in compensation for said dimensional changes thereof.

9. An arrangement for minimizing drift of the frequency generated by an oscillator tube of the resonant cavity type comprising a" device responsive to dimensional changes with operating temperature of the tube cavity structure, a source for continuously supplying cooling fluid to said cavity structure, a member adjustable smoothly to vary the rate of supply of said fluid by said source to said cavity structure, and means operatively connecting said device to saidmember to minimize dimensional changes of said cavity structure.

10. In the operation of an oscillator tube having frequency-determining cavity structure subject to dimensional changes with changes of temperature, the method of minimizing drift of the frequency of the generated oscillations which comprises producing an efiect varying'as a function of the temperature of said cavity structure, and varying the heat-loss from said cavity structure in accordance with the magnitude and sense of the variations of said effect.

11. In the operation of an oscillator tube hav-- ing frequency-determining cavity structure subject to dimensional changes with changes of temperature, the method of minimizing drift of the frequency of the generated oscillations which comprises producing an effect varying as a function of the temperature of said cavity structure, and varying the electrical input to said tube to vary the heat generated therein in accordance with the magnitude and sense of the variations of said effect.

12. In combination with an oscillator tube having frequency-determining cavity structure whose dimensions are affected by ambient conditions and by the electrical input of the tube, a thermal-responsive device in heat-conductive relation with said tube cavity structure and having a member movable in accordance with dimensional changes thereof, and regulating means coupled to said member for adjustment thereby smoothly to vary the loss of heat from said oscillator tube in sense to compensate for frequency changes by minimizing dimentional changes of said tube cavity structure.

13. A combination as in claim 12 in which the thermal-responsive device is attached to said cavity structure externally of the tube for transfer of heat by thermal conduction and in which said regulating means coupled to the movable member of said device is in a circuit controlling the anode-cathode current of the oscillator tube independently of the cathode heating current thereof.

GEORGE L. FERNSLER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,787,300 Alexanderson Dec. 30, 1930 2,027,521 Drake Jan. 14, 1936 2,095,981 Hansell Oct. 19, 1937 2,104,554 Conklin Jan. 4, 1938 2,160,466 Usselman Mar. 30, 1939 2,183,215 Dow Dec. 12, 1939 2,262,044 Philpott Nov. 11, 1941 2,337,214 'Iunick Dec. 21, 1943 2,374,810 Fremlin May 1, 1945 2,442,614 Norton June 1, 1948 2,451,769 Norton Oct. 19, 1948 

